1. Field of the Invention
The present invention relates to a bushing type vibration insulator whose spring constant is changeable, usable as an engine mount, a cab mount, a body mount, a bushing or the like in an automotive vehicle.
2. Description of the Prior Art
For example, a bushing type vibration insulator to be used as a bushing of a suspension of an automotive vehicle requires a high spring constant for the purpose of improving driving stability of the vehicle. On the contrary, such a high spring constant of the bushing causes the vehicle to be uncomfortable to ride in when the vehicle runs on rough road, and additionally tends to provide so-called stuck state of road wheels so that the vehicle becomes difficult to get out of muddy road.
In view of this, a bushing type vibration insulator whose spring constant is variable has been proposed as disclosed, for example, in Japanese Utility Model Provisional Publication No. 60-66507. Such a vibration insulator consists of an elastic member disposed to connect an outer cylindrical member and an inner cylindrical member, the elastic member being made of an elastomeric material such as rubber. Two fluid chambers are formed between the outer and inner cylindrical members in such a manner as to be located in the diametrical direction of the cylindrical members. A part of each fluid chamber is defined by the elastic member. The fluid chambers are adapted to be simultaneously supplied with the fluid at a predetermined pressure by means of a pump and electromagnetic valves which are located remote from the two fluid chambers.
With such a bushing type vibration insulator, its spring constant is raised by increasing the fluid pressures of both the fluid chambers and lowered by decreasing the same fluid pressures. Thus, the vibration insulator improves vehicle comfortableness to drive in particularly during driving on rough road upon lowering the spring constant thereof, while sufficiently effectively preventing the stuck state of the wheels from occurrence.
However, since such a conventional bushing type vibration insulator is disposed remote from the hydraulic pump and the electromagnetic valves, fluid supply and discharge passages for fluidly connecting the vibration insulator and the pump are required to be considerably long. Accordingly, there is a high possibility of fluid leaking from the fluid supply and discharge passages while requiring a large space for installation of the vibration insulator and accessories therefor and extremely increasing the number of steps for production. Furthermore, it is required to provide a separate fluid tank for connecting the pump, the electromagnetic valves and the like.
In addition, because of the long fluid supply and discharge passages, a longer time is necessary to change over the rigidity value of the vibration insulator so as to be impossible to instantaneously change the characteristics of the vibration insulator. Furthermore, the dynamic spring constant of the vibration insulator cannot be suppressed to a lower value owing to line loss.